93 5. Geophysical methods seismic velocity logs extracted from the 3D block at borehole locations M11, M14, M20, and M22. We notice a strong velocity decrease in the 80–90 m depth interval. To quantify porosity variations within this aquifer, the seismic interval velocities were first converted into resistivity values. For this purpose, the empirical relationship between seismic velocity and resistivity proposed by Faust [1953] was used (Mari, Porel & Bourbiaux, 2009). At each well where a long normal log Rt (Chapellier, 2001b) has been recorded, an interval velocity log V was extracted from the 3D block. The two sets of data (resistivity Rt and seismic velocity V) were combined to compute empirical Faust’s laws which were used to transform the 3D pseudo-velocity block into a 3D pseudo resistivity block, using resistivity functions (resistivity logs Rt recorded at 12 wells MP4, MP5, MP6, M11, M13, M14, M18, M21, M04, M07, M08 and M09) as constraints. Figure 6a shows comparisons between resistivity logs Rt from logging and resistivity logs Rt-seis from seismic velocity at boreholes M11 and M14. Figure 6b shows the resistivity logs Rt-seis at Figure 4 3D seismic imaging: in-line 31 pseudo-velocity section (upper part: zoom of 0 to 35 m depth interval). Modified after Mari et al., 2018.
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